458 COMPARATIVE PHYSIOLOGY. 



We must regard the nervous centers as the source of cease- 

 less impulses that operate upon all parts, originating and con- 

 trolling the entire metabolism, of which what we term func- 

 tions are but certain phases, parts of a whole, but essential for 

 the health or nornjal condition of the tissues. Against such a 

 view we know no facts, either of the healthy or disordered or- 

 ganism. 



Summary of Metabolism. — Very briefly and somewhat in- 

 completely, we may sum up the chief results of our present 

 imowledge (and ignorance) as follows : 



Glycogen is found in the livers of all vertebrate and some 

 invertebrate animals. The quantity varies with the diet, being 

 greatest with an excess of carbohydrates. 



Glycogen may be regarded as stored material to be convert- 

 ed into sugar, as required by the organism; though the exact 

 use of the sugar and the method of its disposal are unknown. 



Fat is not stored up in the body as the result of being 

 merely picked out from the blood ready made ; but is a genuiae 

 product of the metabolism of the tissues, and may be formed 

 from fatty, carbohydrate, or proteid food. This becomes es- 

 pecially clear when the difference in the fat of animals from 

 that on which they feed is considered, as well as the direct re- 

 sults of feeding experiments, and the nature of the secretion of 

 milk. 



The liver seems to be engaged in a very varied round of meta- 

 bolic processes ; the manufacture of bile, of glycogen, of urea, 

 and probably of many other substances, some known and 

 others unknown, as chemical individuals. Urea is in great 

 part probably only appropriated by the kidney-cells (Amoeba- 

 like) from the blood in which it is found ready made ; though 

 it may be that a part is formed in these cells, either from 

 bodies some steps on the way toward urea, or out of their pro- 

 toplasm, as fat seems to be by the cells of the mammary gland. 



The leucin (and tyrosin ?) of the digestive canal sustains 

 some relation to the manufacture of urea by the liver, and pos- 

 sibly by the spleen and other organs ; for a proteid diet increases 

 these products, and also the urea excreted. Creatin, one of the 

 products of proteid metabolism, and possibly allied bodies, may 

 be considered as in a certain sense antecedents of urea ; uric-acid, 

 however, does not seem to be such, nor is it to be regarded as a 

 body that has some of it escaped complete oxidation, but rather 

 as a result of a distinct departure of the metabolism ; and there 



